There are increasing opportunities and demands for photonic materials with properties that can be tuned by external stimuli. Colloidal assembly has become one of the most powerful routes for the fabrication of responsive photonic crystals because the stimulus-responsive materials can be incorporated into the colloidal building blocks through chemical synthesis, or they can be integrated into the photonic device during or after the assembly processes. For example, inclusion of superparamagnetic (SPM) components into colloidal building blocks allows for effective control over photonic properties of the assembled structures by using external magnetic fields. Recently, a series of magnetically tunable photonic crystal systems through the assembly of uniform superparamagnetic (SPM) colloidal particles in liquid media with various polarities has been demonstrated. One of the keys of this self-assembly strategy is the establishment of a balance between the magnetically induced dipolar attraction and the repulsions resulted from surface charge or other structural factors. This finely tuned dynamic equilibrium leads to the self-assembly of the magnetic colloids in the form of chain structures with defined internal periodicity along the direction of external field, and also renders the system a fast, fully reversible optical response across the visible-near-infrared range when the external magnetic field is manipulated. To balance the magnetic dipole attractions, a long-range strong repulsive force between SPM particles is demanded to achieve high quality responsive photonic structures in terms of enhancing the structure ordering, and broadening the tunability of interparticle distance and the corresponding diffraction colors. Strong electrostatic repulsions can be established among neighboring Fe3O4 particles by covering their surface with a layer of polyacrylate, a highly charged polyelectrolyte. However, gradual change of the photonic properties over time has been observed, mainly due to the partial detachment of polyacrylate molecules from the particle surface. This drift of photonic performance due to charge instability may limit many of their practical applications. The magnetic assembly process has also been extended to alkanol solvents by modifying iron oxide particle surface with a layer of silica, or nonpolar solvents after further surface modifications. However, although the stability was improved, the electrostatic repulsion as the counterforce to balance the magnetic attraction was diminished. Thus, the tuning range of the diffraction from these photonic assemblies was considerably narrower comparing with that from polyacrylate covered Fe3O4 particles in aqueous solutions.
Accordingly, to ensure a consistent photonic performance over a long period, it is highly desirable to develop methods to maintain a long-term stability of the strong charges on the superparamagnetic particle surface. Furthermore, it would be desirable to have a method and/or process that allow the instantaneous creation of high quality colloidal crystal structures with tailored photonic properties.